1. Field of the Invention
The present invention relates to a photosensitive composition. More particularly, it relates to a photosensitive composition exhibiting excellent adhesion to a support and suitable for a presensitized lithographic printing plate (hereinafter referred to as "PS plate") which is developable with an aqueous alkaline developer and capable of providing a lithographic printing plate showing excellent durability.
2. Prior Art
Most of the photosensitive substances used in presensitized printing materials are diazonium compounds and most commonly used are diazo resins such as a condensate of p-diazodiphenylamine and formaldehyde.
The photosensitive composition comprising diazo resins for producing photosensitive layers of PS plates can be divided into two groups, one of which simply comprises diazo resins, in other words those free of binders as disclosed in U.S. Pat. No. 2,714,066 and the other of which comprises both diazo resins and binders as disclosed in U.S. Pat. No. 4,275,138. However, recently, most of the photosensitive layers of PS plates comprise diazonium compounds and polymers serving as the binder in order to improve physical properties such as printing durability, oleophilicity etc. of the plate.
As such a photosensitive layer, there have been known so-called alkali-developable ones whose un-exposed portion is removed (or developed) with an aqueous alkaline developer as described in U.S. Pat. No. 4,275,138 and so-called solvent-developable ones whose unexposed portion is removed (or developed) with an organic solvent type developer, but the former has become of major interest recently from the viewpoint of safety in operations and health of operators. Such development types of the light-sensitive layer are determined principally depending on properties of binders used. As a method for making binders alkali-soluble, there have been known a method as disclosed in U.S. Pat. No. 4,123,276 which comprises copolymerizing carboxyl group-containing monomers to obtain such a binder and a method as disclosed in U.S. Pat. No. 2,861,058 which comprises reacting hydroxyl groups of polyvinyl alcohol with a cyclic acid anhydride such as phthalic anhydride to introduce carboxyl groups into the resultant polymers. However, the polymers obtained by these method generally exhibit poor abrasion resistance due to the chemical structures thereof, and therefore the lithographic printing plates containing those binder polymers in the photosensitive layer thereof show poor printing durability. By using polyvinyl acetal, it is possible to form a strong coating showing good abrasion resistance; however, this polymer has also a drawback that it can give only the organic solvent developable PS plate.
On the other hand, polyurethane resins have been known as a polymer having excellent abrasion resistance. Combinations of diazonium compounds with substantially linear polyurethane resins and combinations of polycondensates of diazonium salts with branched polyurethane resins are described in U.S. Pat. Nos. 3,660,097 and 4,337,307. However, these polyurethane resins have no alkali-soluble group essentially and therefore has an insufficient solubility in an aqueous alkali developer. It is thus quite difficult to conduct the development in such a manner that the photosensitive layer can be completely removed from the non-image area (exposed area).
Further, combinations of diazonium compounds and polyurethane resins having a carboxyl group are disclosed in U.S. Pat. No. 4,983,491. Although they are soluble in the aqueous alkaline developer and have an excellent abrasion resistance, the adhesion thereof to an aluminum support is not always sufficient. Therefore, there has been problems, for example, the photosensitive layer is sometimes peeled off from the support depending on the storage conditions.